The invention relates to a method and system for estimating the performance of modems and facsimile (“fax”) machines that communicate over a packet-switched network.
Voice over Internet Protocol (“VoIP”) services are widely used today to carry voice telephone conversations over packet-switched networks in digital form. Traditional voiceband modems and fax machines can be connected to such VoIP networks to transmit data over a packet-switched network instead of traditional circuit-switched telephone networks.
It is well known that VoIP services can suffer from performance degradation due to packet loss or packet delay. Service quality monitoring systems have been developed to measure the effects of packet loss or delay on telephone conversations carried over such VoIP networks. These service quality monitoring systems attempt to estimate the subjective experience of end-users speaking over the VoIP network.
Such service quality monitoring (“SQM”) systems typically monitor such things as packet loss rates, packet delay variation levels, and general packet delay times in order to estimate service quality. Further, because packet loss and delay rates usually vary significantly over time, SQM systems often analyze the distribution over time of packet losses and delay rates. SQM systems can convey the results of their service quality analysis as individual parameters such as packet loss rates and packet delay variation. Alternatively, SQM systems can output estimates of subjective end-user quality levels such as a Mean Opinion Score (“MOS”) rating that attempts to encapsulate the end-user's subjective experience on a single scale (such as a 0-100 scale.) Current SQM systems are adapted to measure the effect of packet losses and delays on voice conversations, however, and do not measure the effect of such disruptions on voiceband modems or fax machines that communicate over VoIP networks.
It is well known that voiceband modems and fax machines are very sensitive to packet losses and packet delay variation. When a voiceband modem or fax machine first establishes a connection over a communication channel, the data modulation encoder/decoder (“datapump”) of the modem or fax machine first goes through a training sequence to learn some characteristics of the communication channel in order to evaluate the channel and optimize its transmission speed over the channel. Many users are familiar with the series of tones and hisses produced during the training sequence. After completing the training sequence, the voiceband modem or fax machine will commence sending and receiving data packets over the communication channel.
If, however, the communication channel is unreliable and data packets are lost or unduly delayed, the voiceband modem or fax machine will need to repeat the training sequence in order to re-synchronize its connection with its counterpart on the other end of the communication channel and to re-evaluate the transmission quality of the channel and adapt accordingly. Such a repetition of the training sequence is called the “retrain” process and results in a degradation of service.
At the beginning of the retrain process, the datapump of the modem or fax machine will usually wait a short period to allow the communication channel to recover from its degraded state. Such a waiting period is typically from ½-1 second in length. After the waiting period, the datapump will begin the active portion of the retrain sequence where the data pump re-analyzes the communication channel and re-synchronizes with the other modem or fax machine on the other end of the communication channel. The active portion of the retrain sequence can typically last between 5-15 seconds.
If losses on the communication channel cause further packet losses or undue packet delay during the retrain sequence, then the modem or fax machine will have to terminate the retrain sequence and begin another retrain sequence. If the communication channel is extremely unreliable and the modem or fax machine repeatedly fails to complete the retrain process, then the modem or fax machine will terminate the connection altogether.